The present invention relates to an Fexe2x80x94Crxe2x80x94Al-based alloy foil having oxidation and deformation resistances at high temperatures and to a manufacturing method thereof. The alloy foil is suitable for catalytic converters for automotive exhaust gas purification, where the catalyst carriers and the catalytic converters are exposed to intense vibration and thermal shock in a high-temperature oxidizing atmosphere. The alloy foil is also useful for devices and apparatuses for combustion gas exhaust systems.
Replacing conventional ceramic catalytic converter carriers for automotive exhaust gas purification apparatuses with a metal honeycomb as disclosed in Japanese unexamined patent publication No. 56-96726 facilitates the miniaturization of catalytic converters and improves engine performance.
In view of environmental protection, it is required that automotive exhaust gas purification apparatuses be capable of starting a catalytic reaction immediately after the engine is started. A catalytic converter of the apparatus is located as near the combustion environment as possible so that high temperature exhaust gas can immediately reach the converter, and thus the catalytic converter reaches a catalytic activation temperature in a short period. In this case, the catalytic converter is exposed to thermal cycles of heating and cooling in a high-temperature range and engine judders, that is, it has been used in severe conditions. Ceramics conventionally used as a material for the catalytic converters are not suitable for practical use because they are easily damaged by thermal shock. Thus, oxidation-resistant metals such as Fexe2x80x94Crxe2x80x94Al-based alloys are used. An Fexe2x80x94Crxe2x80x94Al-based alloy exhibits oxidation resistance at high temperatures because easily oxidizable Al is oxidized prior to Fe to form an oxide film of Al2O3 which protects the alloy surface from the oxidation. After the consumption of Al in the alloy, Cr is preferentially oxidized at the interface between the Al2O3 oxide film and the alloy. Such Fexe2x80x94Crxe2x80x94Al-based alloys are disclosed in Japanese unexamined patent publication Nos. 56-96726 (mentioned above), 7-138710, 9-279310, etc.
As mentioned above, emission control is strengthened in view of environmental protection, the demand that exhaust gas be purified from the beginning of the engine start has been intensifying in these years. In order to comply with the control, the use of a metal carrier has been increasing, and the demand for thin foil thereof is intensifying. This is because a reduction in the wall thickness of the metal carrier allows exhaust back pressures to be reduced and allows the catalyst to be activated in a short period due to decreased heat capacity. However, the reduced foil thickness requires that materials for the foil have a higher oxidation resistance. Also, since the reduced foil thickness leads to deformation by heat, deformation resistance at high temperatures (less elongation at high temperatures and less fracture due to heat stress) is further required.
Conventional Fexe2x80x94Crxe2x80x94Al-based alloys have a deformation problem at high temperatures and improved oxidation resistance is required to help to improve the durability thereof. The present invention is intended to provide an Fexe2x80x94Crxe2x80x94Al-based alloy for catalyst carriers and a foil thereof having a thickness of 40 xcexcm or less, the alloy and the foil improved in the oxidation resistance at high temperatures and having excellent deformation resistance. The material of the present invention is specifically suitable for catalytic converter materials and for instruments and apparatuses in combustion gas exhaust systems.
The inventors have found that the effective content of La depends on the foil thickness through close examinations of the contents of La, Zr, and Hf, the initial oxidation resistance, and the deformation resistance at high temperatures. The inventors reached a result that the thinner the foil thickness is, the more remarkable the effect is, and thus the present invention was completed.
A first aspect of the invention is an Fexe2x80x94Crxe2x80x94Al-based alloy foil comprising 0.07 mass % or less of C, 0.5 mass % or less of Si, 0.5 mass % of Mn, 16.0 to 25.0 mass % of Al, 0.05 mass % or less of N, La, Zr, and the balance being Fe and incidental impurities. The contents by mass % of La and Zr meet the following ranges when the foil thickness thereof is t xcexcm:
1.4/txe2x89xa6Laxe2x89xa66.0/txe2x80x83xe2x80x83(1)
xe2x80x830.6/txe2x89xa6Zrxe2x89xa64.0/txe2x80x83xe2x80x83(2)
A second aspect of the invention is the Fexe2x80x94Crxe2x80x94Al-based alloy foil according to the first aspect, further comprising Hf and the balance being Fe and incidental impurities, wherein the content by mass % of La, Zr, and Hf meet the following ranges:
1.4/txe2x89xa6Laxe2x89xa66.0/txe2x80x83xe2x80x83(1)
0.4/txe2x89xa6Zrxe2x89xa62.0/txe2x80x83xe2x80x83(3)
0.5/txe2x89xa6Hfxe2x89xa62.0/txe2x80x83xe2x80x83(4)
A third aspect of the invention is the Fexe2x80x94Crxe2x80x94Al-based alloy foil according to the first or the second aspects in which the final foil thickness is preferably 40 xcexcm or less. A fourth aspect of the invention is the Fexe2x80x94Crxe2x80x94Al-based alloy foil according to the first, the second, or third aspects, further comprising lanthanoids other than La and Ce such that the contents thereof are each 0.001 to 0.05 mass % and totally 0.2 mass % or less. Such an alloy foil has excellent characteristics.
A fifth aspect of the invention is a favorable Fexe2x80x94Crxe2x80x94Al-based alloy foil according to the first to fourth aspects, in which the completed foil preferably has a structure of which the mean crystal grain size is 5 xcexcm or less or a rolling structure. A sixth aspect of the invention is a method of manufacturing an Fexe2x80x94Crxe2x80x94Al-based alloy foil. The manufacturing method comprises preparing a molten steel comprising 0.07 mass % or less of C, 0.5 mass % or less of Si, 0.5 mass % of Mn, 16.0 to 25.0 mass % of Cr, 1 to 8 mass % of Al, 0.05 mass % or less of N, La, Zr, and the balance being Fe and incidental impurities in a molten state. The method also comprises: pouring the molten steel into a slab; perform hot rolling; perform annealing; and repeating cold rolling and annealing to form a foil. In this instance, the contents by mass % of La and Zr meet the following ranges when the foil thickness thereof is t xcexcm:
1.4/txe2x89xa6Laxe2x89xa66.0/txe2x80x83xe2x80x83(1)
0.6/txe2x89xa6Zrxe2x89xa64.0/txe2x80x83xe2x80x83(2)
A seventh aspect of the invention is the manufacturing method of an Fexe2x80x94Crxe2x80x94Al-based alloy foil according to the sixth aspects, in which the molten steel further comprises Hf and the contents by mass % of La, Zr, and Hf meet the following ranges:
1.4/txe2x89xa6Laxe2x89xa66.0/txe2x80x83xe2x80x83(1)
0.4/txe2x89xa6Zrxe2x89xa62.0/txe2x80x83xe2x80x83(3)
0.5/txe2x89xa6Hfxe2x89xa62.0/txe2x80x83xe2x80x83(4)
An eighth aspect of the invention is the manufacturing method of an Fexe2x80x94Crxe2x80x94Al-based alloy foil according to the sixth or seventh aspects, in which annealing before the final cold rolling is performed at a temperature of 700 to 1000xc2x0 C.
In the manufacturing method of the Fexe2x80x94Crxe2x80x94Al-based alloy foil of the present invention, the annealing before the final cold rolling is performed at a temperature of 700 to 1000xc2x0 C. in the foil production process.